1. Field of the Invention
The present invention relates to a semiconductor integrated circuit, and more particularly, to a reference voltage generator capable of stably generating a reference voltage regardless of temperature variations.
2. Discussion of Related Art
In general, a reference voltage can be used as a threshold voltage against which data is compared to determine the logic level of the data. If the voltage of the data is lower than the reference voltage, the logic level of data is logic xe2x80x9clowxe2x80x9d. If the voltage of the data is higher than the reference voltage, the logic level of data is logic xe2x80x9chighxe2x80x9d. Accordingly, if the reference voltage is varied, the logic level of data is also varied.
FIG. 1 is a circuit diagram of a conventional reference voltage generator. Referring to FIG. 1, a reference voltage generator 100 includes a voltage bias unit 101, a voltage controller 103, and a capacitor 105. The voltage bias unit 101 includes a voltage divider which outputs a reference voltage VREF, the voltage value is determined by resistors R1xcx9cR5 and a first group of transistors M1xcx9cM20 serially connected between a power supply voltage VDD and a ground voltage VSS. The reference voltage VREF is outputted at the node between a first resistor R1 and secondxcx9cfifth resistors R2xcx9cR5 and a first group of transistors M1xcx9cM20.
The voltage controller 103 controls the reference voltage VREF using a second group of transistors M31 and M32 connected between the reference voltage VREF and the ground voltage VSS. The second group of transistors M31 and M32 are turned on/off by a voltage applied at their gates at node A. The voltage applied at the node A can be varied by programming fuses F1, F2, and F3, in which each fuse acts as a short-circuit to bypass each of the third, fourth, and fifth resistors R3, R4, and R5, respectively, unless the fuse is cut or blown. When the second group of transistors M31 and M32 are turned on, the reference voltage VREF is pulled toward VSS and therefore the voltage is decreased. When the second group of transistors M31 and M32 are turned off, the reference voltage VREF is maintained at the voltage level based on the voltage divider configuration of the voltage bias unit 101. The capacitor 105 is charged by the reference voltage VREF for maintaining the reference voltage VREF to each circuit connected to the reference voltage VREF. The reference voltage is expressed by Formula (1):                     VREF        =                  Vtp          ⁡                      (                          1              +                              Rch                R1                                      )                                              (        1        )            
where Vtp indicates the threshold voltage of the second group of transistors M31 and M32, Rch indicates the channel resistance of the first group of transistors M1xcx9cM20, and R1 indicates the resistance of the first resistor R1 of the voltage bias unit 101.
In the reference voltage generator 100, if the power supply voltage VDD is decreased, the reference voltage VREF is also decreased. If the reference voltage VREF is decreased, the variation range of the reference voltage VREF due to temperature variations is widened. It is known that Rch varies to a much larger extent as compared to Vtp and R1 when temperature is varied. In other words, the variation of the reference voltage VREF due to temperature variation is based largely on the variation of Rch.
It is an object of the present invention to provide a reference voltage generator capable of stably generating a reference voltage regardless of temperature variations.
According to an aspect of the present invention, a reference voltage generator comprises a voltage bias unit having a plurality of resistors and a first group of transistors serially connected between a power supply voltage node and a ground voltage node, and a preliminary reference voltage node connected to one of the plurality of resistors or transistors to produce a preliminary reference voltage; a voltage controller connected between the preliminary reference voltage node and the ground voltage node to adjust the preliminary reference voltage; a temperature compensator having a second group of transistors serially connected between the preliminary reference voltage node and a reference voltage node to compensate for temperature variation and produce a reference voltage; and a voltage compensator having a third group of transistors serially connected between the reference voltage node and the ground voltage node for controlling the reference voltage.
In the reference voltage generator, each of the gates of the second group of transistors is connected to the ground voltage node and a source or drain selectively short-circuited. Each of the gates of the third group of transistors is connected to the preliminary reference voltage node and a source or drain selectively short-circuited.
According to another aspect of the present invention, a reference voltage generator comprises a voltage bias unit having a first group of resistors and a first group of transistors serially connected between a power supply voltage node and a ground voltage node, and a preliminary reference voltage node connected to one of the first group of resistors or the first group of transistors to produce a preliminary reference voltage; a voltage controller connected between the preliminary reference voltage node and the ground voltage node to control the preliminary reference voltage; a temperature compensator having a second group of resistors serially connected between the preliminary reference voltage node and a reference voltage node; and a voltage compensator having a second group of transistors serially connected between the reference voltage node and the ground voltage node. The voltage compensator and the temperature compensator are connected to compensate for temperature variation and produce a reference voltage.
Advantageously, the reference voltage generator according to the present invention can stably generate a reference voltage by minimizing the variation of the reference voltage with respect to temperature variations.